<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title></title>
<style>
/*方法一css*/
body {
  background: gray
}

.iframe {
  position: relative;
  width: 100px;
  overflow: hidden;
  margin-top: 40px
}

.circular {
  width: 200px;
  height: 200px;
  border-radius: 50%;
}

.one {
  background: red;
}

.two {
  background: gray;
  position: absolute;
  top: 35px;
}

.triangle {
  width: 0;
  height: 0;
  border-top: 60px solid gray;
  border-left: 70px solid red;
  border-bottom: 60px solid gray;
  position: absolute;
  top: 0;
  left: 100px;
}
/*方法二css*/

#test1{
    margin: 100px;
    border-radius: 100px 1px;
    border-top: 40px solid #f00;
    border-left: 0px solid #fff);
    width: 100px;
    height: 100px;
}

#test1::after{
    content: '';
    display: block;
    width: 0px;
    height: 0px;
    border: 50px solid transparent;
    border-left-color: #f00;
    margin: -70px 100px;
}
</style>
</head>
<body>    
<!-- 方法一 -->
<div class="iframe">
    <div class="circular one"></div>
    <div class="circular two"></div>
</div>
<div class="triangle"></div>
<!-- 方法二 -->
<div id="test1"></div>
</body>
</html>